During the completion of a well which has been successfully drilled to a producing formation, it is necessary to perforate into the formation. Perforations are formed by shaped charges. One procedure for positioning shaped charges in the well borehole is to suspend an assembly known as a tubing conveyed perforating gun assembly. The TCP assembly is lowered into the borehole on a tubing string, and a weight is normally dropped down the tubing to land on the top of the TCP apparatus. This detonates the explosive charge equipment at the top of the TCP assembly and initiates burning of a primer cord extending along the shaped charges. The number of shaped charges can be varied widely. It is conceivable that the shaped charges will number in the hundreds, requiring a very long primer cord.
After the equipment is suspended several thousand feet deep in a well, the operator then normally drops the weighted bar in the tubing to initiate detonation. Detonation is fairly easy to detect at the surface. There is, however, always a sense of uncertainty of whether or not all the shaped charges have been detonated. At the time of retrieving the TCP gun assembly, the tubing is first retrieved. Then, the spent shaped charges in the support assembly is retrieved. This creates a dangerous moment at the wellhead when the gun assembly is pulled out of the well. This exposes personnel to great danger in the event that one of the shaped charges did not detonate. One mode of failure results from interruption of the primer cord. It is perhaps the most common mode of failure in shaped charge detonation. That is, the shaped charges at the top end of the assembly are all detonated. Several bottom located shaped charges are not detonated. This may happen with only a few shaped charges or may involve a very large number of large number of shaped charges. The failure to detonate all the shaped charges creates a significant hazard on retrieving the assembly from the well borehole.
It has been discovered that the last shaped charge in the string, the charge located at the bottom of the string, is the shaped charge to be investigated. Of all the charges in the string, this is the particular shaped charge that is most likely involved in the event there is a failure in the equipment at any point in the assembly. This is true whether or not the assembly comprises only a few shaped charges or several hundred shaped charges. That is, when the last charge is detonated, great certainty is obtained; if the last charge is not detonated, this is indicative of serious problems.
The present apparatus is a detection system to be used with the last charge. Assume that the number of shaped charges required for the string is N where N is a whole number integer. An N+1 shaped charge is added to the string. However, it is not intended to form a perforation in the ordinary operation of the shaped charge assembly. The shaped charge which comprises N+1 charge is used as a signaling charge. It is fired straight down the well borehole for the purpose of interpreting a signal transmitted to the surface. This signal is detected at the surface. The signal presence determines whether or not the N+1 shaped charge has been detonated. If it detonated, this is an indication that the entire string of shaped charges properly detonated. If the transmitted signal persists, this warns the personnel retrieving the shaped charge assembly that live shaped charges remain in the assembly. The risk is markedly clarified.
While the foregoing briefly summarizes the present disclosure, more details will be determined on review of the written specification found below in conjunction with drawings.